Parkinson's Disease (PD) is a motor disorder brought on by neurodegeneration of midbrain dopamine neurons. However, a subpopulation of these cells is relatively spared both in PD and in PD animal models - those that project from the medial ventral tegmental area (VTA) to the ventromedial striatum (VMS or the medial shell of the nucleus accumbens). This projection also contains many dopamine neurons that express the vesicular glutamate transporter (VGLUT2) and co-release the excitatory neurotransmitter glutamate. Because VGLUT2 and the vesicular monoamine transporter (VMAT2) appear to localize to an overlapping population of synaptic vesicles in the VMS, the vesicular co-entry of glutamate may confer neuroprotection by increasing the vesicular pH gradient (DpH). Larger DpH would then be predicted to increase the vesicular storage of dopamine, serving to sequester dopamine and other potentially cytotoxic VMAT2 substrates into vesicular bodies where their toxicity is mitigated. The first aim of this proposal will test whether VGLUT2 confers neuroprotective benefits in an MPTP lesion model of PD using selective conditional knockout mice and viral expression strategies. Following the loss of dopamine neurons in PD or PD lesion models, output from the dopaminoceptive medium spiny neurons (MSN) in the basal ganglia is dysregulated. In particular, over-activation of the dopamine D2 receptor containing MSNs of the indirect pathway and consequent over-excitation of the subthalamic nucleus appear to account for the most severe motor symptoms associated with PD. We will thus test whether flipping the polarity of indirect pathway output from inhibitory to excitatory can restore motor behaviors in a PD lesion model. These studies will test novel hypotheses and a new molecular target (VGLUT2) in the neural circuitry that contribute to PD. The work will provide important information about the plasticity of these circuits to changes in excitatory transmitter release, an may lead to new approaches for the treatment and prevention of Parkinson's Disease.